A light detection method utilizing a quantum well with wavelength selectivity is recited in, for example, Appl. Phys. Lett. Vol. 47, No. 3, (1985), pp. 190 to 192. The method is based on the principle which will be described.
The quantum well structure exhibits exciton absorption at room temperature and the absorption characteristics thereof are sharp.
FIG. 4 shows photocurrent spectra in case where various reverse bias voltages are applied to the quantum well structure. In FIG. 4, the relation of the reverse bias voltages V.sub.0, V.sub.1, and V.sub.2 is V.sub.0 &lt;V.sub.1 &lt;V.sub.2.
As shown in FIG. 4, when a reverse bias voltage is applied to a quantum well structure, the absorption peak of incident light in the quantum well layer shifts toward the longer wavelength side because of the quantum confined Stark effect. Accordingly, an external electric field which is applied to the structure shifts the absorption peak to the wavelength to be detected, so that the light detection can be performed with high wavelength selectivity.
Furthermore, the incident light can be resolved into wavelengths .lambda..sub.1 and .lambda..sub.2 as shown in FIG. 4 utilizing the wavelength selectivity of the quantum well structure.
FIG. 5 is a diagram showing a construction of Self-electro-optic effect element and an external circuit driving the same, which is recited in, for example, Appl. Phys. Lett., Vol. 45, No. 1 (1984) pp. 13 to 15.
In FIG. 5, reference numeral 51 designates a GaAs/AlGaAs multi-quantum well region which is between undoped AlGaAs layers 52. A p type AlGaAs layer 53 and an n type AlGaAs layer 54 are disposed sandwiching the undoped layers 52. This p-i-n structure is disposed on an n type GaAs substrate 55 having an aperture in the central portion. An electrode 56 is produced on a rear surface of the substrate 55 and on the p type AlGaAs layer 53. An aperture for incident light is provided on the electrode 56 on the p type AlGaAs layer 53. An external power supply 57 and a resistor 58 are connected in series between the electrodes 56 as an external circuit.
FIG. 12 shows photocurrent spectra for three applied voltages (V.sub.0 &lt;V.sub.1 &lt;V.sub.2) when the external resistor 58 is not connected (R=0.OMEGA.). FIG. 13 shows photocurrent response against applied voltage at the wavelength .lambda..sub.1. The quantum well structure generally has an absorption peak that shifts toward the longer wavelength side, as shown in FIG. 12, with an increase in the external applied voltage Vex, as described above.
Next, a description will be given of the operation.
When the light signal of the wavelength .lambda. and intensity Pin is input from the aperture of the electrode 56, the photocurrent flows in accordance with the absorption characteristics of the element including the quantum well structure. The characteristics in a case where the external resistor 58 is not connected to the element are as those shown in FIGS. 12 and 13. On the other hand, when the external resistor 58 is connected to the element, a voltage drop IR arises induced by the photocurrent I, modulates the voltage applied to the element. Thus, also the photocurrent I is again modulated.
The load characteristics are represented by the following formula: ##EQU1## where C is a constant.
The intersection between the light response curve of FIG. 13 and the straight line represented by the formula (1) results in a solution. The load characteristics for two incident light powers P.sub.1 and P.sub.2 (P.sub.1 &lt;P.sub.2) are shown by dotted straight lines in FIG. 13, and the response characteristic of this element (incident light power-photocurrent characteristic) is as shown in FIG. 14, resulting in a bistable property in a range between P.sub.1 and P.sub.2.
Similarly, the bistable property can be obtained in the characteristics of the photocurrent against the applied voltage V.sub.ex, the external resistor R, and the wavelength .lambda..
The above-described prior art light detection method using a quantum well structure having wavelength selectivity utilizes the absorption spectrum and has a problem in its resolution.
Furthermore, the hybrid type optical bistable element of FIG. 5 takes only two stable states in accordance with external parameters such as light input, applied voltage, external resistor, and wavelength.